An organic electroluminescence (EL) display device enables full color display and thinner design and therefore is expected to be applied to a display device. However, the organic EL display device has various drawbacks resulting from the injection of electric charges. For example, since a direct-current voltage is applied to the organic EL display device for driving, this causes the problem of shortening the service life of the device because impurities accumulate on one electrode. To overcome these drawbacks, a display device utilizing electrochemi-luminescence (ECL) has been developed, wherein a chemical reaction is induced by applying a voltage to the display device, thereby enabling the display device to chemically emit light. Since the display device utilizing ECL (hereinafter, referred to as ECL display device) can also be driven by an alternating-current voltage, the above problem can be avoided.
Since a luminescent layer of the ECL display device includes a liquid having fluidity, a luminescent material included in the luminescent layer is liable to circulate as compared with an organic EL display device where a luminescent layer is constituted by a solid material. Therefore, a fixed point defect, such as burn-in, is less liable to occur and the ECL display device generally has high reliability. Further, the ECL display device has a mono-layer structure which includes a solution including the luminescent material producing ECL, and electrodes for applying a voltage to the solution. Unlike the organic EL display device, the ECL display device need not laminate a charge transporting layer on the electrodes. Therefore, the ECL display device can be manufactured at lower cost. Furthermore, the ECL display device can be driven at lower voltage according to the principle based on electrochemical reactions.
In the case of an ordinary display device, the optical characteristic thereof can be controlled by applying a potential difference between two electrodes. On the other hand, in the case of the ECL display device, since an electrochemical reaction is utilized to emit light, it is required to control the optical characteristic thereof by accurately applying an oxidation-reduction potential of the luminescent material to electrodes. In the case of a two-electrode system, when a potential difference is applied between electrodes, it is impossible to directly measure an electric potential at each of the electrodes. Therefore, to apply an appropriate electric potential to each of the electrodes, it is required to provide a reference electrode which indicates a specific electrode reaction and has well-known electric potential, and control the electric potential of each electrode by measuring the potential difference between the reference electrode and each electrode.
JP-A 10-135540 (KOKAI) discloses an electrochemi-luminescence cell including a solution of an electrochemi-luminescent material, electrodes for applying an electric potential to the solution, and a reference electrode which is maintained to have a reference potential. However, JP-A 10-135540 (KOKAI) has not concretely described a method of driving the electrochemi-luminescence cell.
Such a conventional display device has the problem of permitting luminance to decrease, be made uneven, and the like when the polarity of a voltage is reversed. Therefore, it is required that there will be provided a display device which is capable of stable potential control and whose pixel units are reliable and a method of driving the display device.